(1) Field of the Invention
The present invention relates to, in an air/fuel ratio control device for an internal combustion engine for feedback control of an air/fuel ratio of mixture sucked into an engine based on oxygen concentration in exhaust gas detected by an oxygen sensor, a method for diagnosing deterioration in the above oxygen sensor.
(2) Related Art of the Invention
The following device is known as a fuel supply control device for an engine having a feedback control function for an air/fuel ratio.
That is, and oxygen sensor for outputting a detection signal at a level corresponding to oxygen concentration in exhaust gas is provided in an exhaust system of an engine, and it is judged whether an actual air/fuel ratio is richer or leaner than a target air/fuel ratio by comparing the detection signal from the above oxygen sensor with a reference level corresponding to the target air/fuel ratio. And an air/fuel ratio feedback correction coefficient for multiplying a basic fuel supply amount calculated from a detection result of an intake air amount is controlled in the direction where the actual air/fuel ratio gets close to the target air/fuel ratio based on the above rich/lean judgement so that the target air/fuel ratio is stably obtained (See Japanese Unexamined Patent Publication No. 60-240840).
In the meantime, with such a device that carries out the above air/fuel ratio feedback control as above, when the oxygen sensor deteriorates and its output characteristic changes, even if the feedback control is carried out toward the target air/fuel ratio on control, the actual air/fuel ratio is deviated from the target air/fuel ratio, which makes a problem.
Then, a device for diagnosing deterioration of the oxygen sensor has been proposed such as a device which measures a cycle of the detection signal of the oxygen sensor during the air/fuel feedback control and diagnoses deterioration of the oxygen sensor (deterioration in response speed) based on changed in this cycle.
However, with the above deterioration diagnosis based on the detection signal cycle, the cycle might be changed by the causes other than deterioration in response in the oxygen sensor such as increase in valve deposit or change in fuel wall flow rate changed by intake manifold temperature or carburetion of fuel, which leads to the problem that high diagnosing accuracy is hard to be maintained.
In this respect, a device, as disclosed in the Japanese Unexamined Patent Publication NO. 62-78444, which diagnoses response of the oxygen sensor by measuring a time interval (response time) with which an actual detection signal crosses two reference detection signals is not hardly affected by change in the above valve deposit or fuel wall flow rate, and diagnosing accuracy is secured.
However, in order to measure the time when the reference detection signals are crossed with high accuracy, it is necessary to set the interval between the reference detection signals wide enough to lengthen the time to be measured, but when the oxygen sensor deteriorates, amplitude of not only the response but of the oxygen sensor output might be reduced, which makes it necessary to set the interval between the above reference detection signals narrower to surely measure the response time even if the amplitude of the output is reduced, and it was difficult to surely measure the response time and moreover, with high accuracy.